The Echinoid Directory

Biology and geological history of spatangoids

Biology

Spatangoids are benthic deposit feeders. Most live infaunally, buried as much as 15-20 cm beneath the surface. Those living in relatively coarse and porous sediments do not have any specialisation for infaunal life other than their dense aboral spines and aboral respiratory tube feet. However, those living in finer, more impermeable sediments use their adapical tube-feet in the frontal ambulacrum to build and maintain an open passageway to the surface. They also have a finer and more uniform coat of aboral spines that maintain the burrow wallls. Most also have fascioles, which are specialised bands of fine spines that are densely ciliated and have mucous glands at their tips. The function of fascioles is two-fold: to generate water circulation within the burrow and to produce a sheet of mucous that can be used to cement the walls of the burrow and prevent fine sediment from falling in between the spines.

Spatangoids feed using their oral tube-feet to pick up and transfer sediment into the mouth. These tube feet are penicillate, that is their disc is covered in a mass of tiny finger-like projections. The tube-feet are wiped against an internal calcite bar, termed the raker, which lies just inside the peristome to remove particles. Those with deep frontal grooves feed primarily on particles that fall down the apical shaft into the burrow. These become trapped in a mucous belt that floors the frontal groove and is constantly drawn downwards and into the mouth.

The sexes are separate and a number of groups have become lecithotrophic and brood their young in specially sunken parts of the test.

Geological history

The earliest spatangoid is Toxaster africanus from the Valanginian near the base of the Cretaceous. Spatangoids therefore appear slightly later than their sister group the holasteroids. In the Lower Cretaceous toxasterids and hemiasterids diversified in continental shelf clastic settings. By the Upper Cretaceous the earliest members of the Micrasterina and Paleopneustina had diverged, and spatangoids expanded into Chalk seas. Hemiasterinids and micrasterinids dominated the faunas of the Upper Cretaceous, with paleopneustinids starting to become important towards the end of the Cretaceous. Toxasterids continue through to the end of the Cretaceous but did not survive into the Tertiary.

The Tertiary saw a major diversification of both paleopneustinids and micrasterinids, with the great majority of extant families in existence by the end of the Eocene. By contrast hemiasterids and the Cretaceous lineage of micrasterids went into decline and today are known only from deep-water settings. Spatangoids are probably as diverse now as they have ever been.